U.S. patent number 6,697,491 [Application Number 09/146,442] was granted by the patent office on 2004-02-24 for 5-2-5 matrix encoder and decoder system.
This patent grant is currently assigned to Harman International Industries, Incorporated. Invention is credited to David H. Griesinger.
United States Patent |
6,697,491 |
Griesinger |
February 24, 2004 |
5-2-5 matrix encoder and decoder system
Abstract
A sound reproduction system has been developed, for converting
signals on two input channels into surround signals on five or
seven output channels and vice-versa. A decoder is included which
enhances the correlated component of the input signals in the
desired direction and reduces the strength of such signals in
channels not associated with the encoded direction, while
preserving the apparent loudness of all output channels, the
separation between the respective left and right output channels
and the total energy of the uncorrelated component of the input
channels in each output channel. Included within the decoder is a
uniquely defined matrix that helps to ensure that the surface of
the output signals is smooth and continuous. An encoder is also
included which encodes five or seven channels of sound into two so
the two channels may be decoded by a variety of decoders with the
correct sound direction and level.
Inventors: |
Griesinger; David H.
(Cambridge, MA) |
Assignee: |
Harman International Industries,
Incorporated (Northridge, CA)
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Family
ID: |
31497829 |
Appl.
No.: |
09/146,442 |
Filed: |
September 3, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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684948 |
Jul 19, 1996 |
5796844 |
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Current U.S.
Class: |
381/20;
381/18 |
Current CPC
Class: |
H04S
3/02 (20130101); H04S 7/307 (20130101); H04S
2400/05 (20130101); H04S 2420/01 (20130101) |
Current International
Class: |
H04S
5/02 (20060101); H04S 5/00 (20060101); H04S
3/02 (20060101); H04S 3/00 (20060101); H04R
005/00 () |
Field of
Search: |
;381/18-23,1,119,307,27 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0782372 |
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Feb 1997 |
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EP |
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0 533 757 |
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Sep 1998 |
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EP |
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8-51698 |
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Feb 1996 |
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JP |
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WO9119407 |
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Dec 1991 |
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WO |
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WO9215180 |
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Sep 1992 |
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WO |
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Other References
Notice of Submission of Opinion for corresponding Korean Patent
Application No. 7002377/2000, dated Jun. 30, 2003, 2 pages.
(translation and original submitted)..
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Primary Examiner: Nguyen; Duc
Assistant Examiner: Lao; Lun-See
Attorney, Agent or Firm: Brinks Hofer Gilson & Lione
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Patent
Application No. 60/058,169, entitled "5-2-5 Matrix Encoder and
Decoder System" filed Sep. 5, 1997; and is a continuation-in-part
of U.S. patent application No. 08/684,948, entitled "Multichannel
Active Matrix Sound Reproduction with Maximum Lateral Separation"
filed Jul. 19, 1996 (now issued U.S. Pat. No. 5,796,844).
Claims
What is claimed is:
1. A surround sound decoder for redistributing a pair of left and
right audio input signals including directionally encoded and
non-directional components into a plurality of output channels for
reproduction through loudspeakers surrounding a listening area, and
incorporating means for determining the directional content of said
left and right audio signals and generating therefrom at least a
left-right steering signal and center-surround steering signal, the
decoder comprising: left and right input terminals for receiving
said corresponding left and right audio input signals; left and
right delay means for producing delayed left and right audio
signals from said left and right audio input signals; a plurality
of multiplier means equal to twice the number of said plurality of
output channels, organized in pairs, a first element of each said
pair receiving said delayed left audio signal and a second element
receiving said delayed right audio signal, each of said multiplier
means multiplying its input audio signal by a variable matrix
coefficient to provide an output signal; said variable matrix
coefficient being controlled by one or both of said steering
signals; and a plurality of summing means one for each of said
plurality of output channels each said summing means receiving the
output signals of a pair of said multiplier means and producing at
its output one of said plurality of output signals, the decoder
having said variable matrix values so constructed as to reduce
directionally encoded audio components in outputs which are not
directly involved in reproducing them in the intended direction and
enhance directionally encoded audio components in the outputs which
are directly involved in reproducing them in the intended direction
so as to maintain constant total power for such signals, while
preserving high separation between the left and right channel
components of non-directional signals regardless of the said
steering signals, and maintaining the loudness defined as the total
audio power level of non-directional signals effectively constant
whether or not directionally encoded signals are present and
regardless of their intended direction if present.
2. The decoder of claim 1 wherein said plurality of output channels
is five, identified as left front, center, right front, left
surround and right surround.
3. The decoder of claim 2 further comprising frequency-dependent
variable filter means following said left surround and right
surround outputs so as to vary the frequency response and phase
response of the outputs in a prescribed manner said variation being
controlled by a number of control signals responding to the
presence of surround or background ambience components detected in
said left and right audio input signals.
4. The decoder of claim 2 further comprising frequency-dependent
variable filter means and additional delay means following said
left and right surround outputs for providing from each said
surround output a side and a rear output channel such as to vary
the frequency and phase responses of the several outputs in a
prescribed manner said variation being controlled by a number of
control signals responding to the presence of surround or
background ambience components detected in said left and right
audio input signals.
5. The decoder of claim 3 or 4 wherein said control signals are: a
center-surround control signal responsive to the ratio of in-phase
center signal components to surround or antiphase signal components
contained in said left and right audio input signals; and a
background control signal responsive to the presence of antiphase
signal components contained in said left and right audio input
signals during periods when no strongly steered signals are
present.
6. The decoder of claims 1, 2, 3 or 4 wherein at least two
different modes of operation are provided and wherein the matrix
coefficients are controlled differently by said steering signals in
the different modes of operation.
7. The decoder of claim 6 wherein a film mode of operation is
optimized for reproduction of surround-encoded audio signals
derived from film soundtracks and other video sources, and a music
mode of operation is optimized for reproduction of musical
recordings or broadcasts.
8. The decoder of claim 7 wherein for decoding film sources the
matrix values for the left and right front outputs are so
constructed to eliminate as much as possible the center component
of the input signals, and the matrix values for the center output
are so constructed that the attenuation of the center output starts
at least 4 dB greater than previously standard decoders, and
reduces rapidly as the center/surround steering signal becomes more
positive, the intermediate matrix values being determined by the
requirement of keeping the power ratio of the center component to
the uncorrelated component of the input signals identical at the
outputs of the decoder.
9. The decoder of claim 7 wherein for decoding music sources the
matrix values for the center output are so constructed that the
center attenuation starts at least 4 dB greater than a standard
decoder, and reduces gradually to the maximum value for a standard
decoder, a value reached at a center/surround steering signal value
of about 20 degrees, the attenuation then holding relatively
constant as the steering value increases, and where the left and
right front matrix values are so constructed that the center
component of the input signals is not maximally removed from these
outputs, but is deliberately adjusted to preserve at the output of
the decoder the power ratio of the center component to the
uncorrelated component of the input signals, the action of the
center and left and right front elements being additionally limited
at the center/surround steering value that results in an
approximately 6 dB difference in level between the center output
and either the left or right front output.
10. The decoder of claim 1 where the left and right front matrix
elements are so constructed that an input signal encoded to the
rear, such that the direction lies between the left rear direction
and right rear direction, produces no output from the front
outputs.
11. The decoder of claim 1 where the left and right front matrix
elements are so constructed that there is a level boost of about 3
dB for signals that have no net left/right component, but have a
center/surround steering value of about 22 degrees, said level
boost reducing to zero as the center/surround steering value
decreases to zero, increases to 45 degrees, or as the left/right
steering value increases from zero to +-45 degrees.
12. The decoder of claim 3 or 4 further comprising additional
circuitry that creates a background control signal by detecting the
direction of the background sound between notes or syllables in the
input material, said background control signal rapidly being set to
the value of said center-surround steering signal when the
center-surround steering signal is negative, and said background
control signal being slowly set positive when the direction of the
background sound between notes and syllables is in the forward
direction, said background control signal tending to hold a
negative value when surround encoded material is played, and a
positive or zero value when standard two channel material is
played.
13. The decoder of claim 12 where the said background control
signal is used to control the relative loudness of the front and
the rear outputs, such that the loudness of the rear outputs is
reduced when the background between notes is either neutral or
positive in direction.
14. The decoder of claim 12 where the said background control
signal controls a variable low pass filter in the rear outputs such
that the cutoff frequency is set to a user adjustable value when
the background direction signal is positive or zero, and to rises
to a high value when the background direction signal is negative,
thus making the surround outputs less obtrusive when ordinary two
channel material is played.
15. The decoder of claim 12 wherein the said background control
signal controls a variable shelf filter, such that when the
background control signal is positive or zero frequencies above 500
Hz in the rear outputs are attenuated by a user adjustable value,
and when the background control signal is negative this attenuation
is reduced to zero, thus making the surround outputs less obtrusive
when ordinary two channel material is played.
16. The decoder of claim 5 where the rear outputs of the matrix are
split into a side output and a rear output by a combination of
additional delay in the rear output, and a variable low pass filter
in the side output, said low pass filter being set to a high
frequency when the said center-surround steering signal is more
positive than -22 degrees, and as the center-surround steering
signal becomes more negative than -22 degrees the low pass
frequency rapidly being reduced, to a final value of 500 Hz when
the center-surround steering signal reaches its minimum value of
-45 degrees.
17. The decoder of claim 4 where the left and right surround
outputs of a five channel version of the decoder are additionally
supplied with a variable filter that emulates the frequency
response of the human head/pinnae system for sound sources that are
more than 150 degrees in azimuth from the front, such that when the
said center-surround steering signal is zero or positive the filter
has its maximum effect, and as the center-surround steering signal
varies from zero to -15 degrees the filter action is reduced to
zero, and as the center-surround steering become further negative
the filter once again acts maximally, and then modifies itself
slightly to correspond to the frequency response of the human
head-pinnae system for sound sources fully to the rear as the
center-surround steering signal reaches its minimum value of -45
degrees.
Description
FIELD OF THE INVENTION
This invention relates to sound reproduction systems involving the
decoding of a stereophonic pair of input audio signals into a
multiplicity of output signals for reproduction after suitable
amplification through a like plurality of loudspeakers arranged to
surround a listener, as well as the encoding of multichannel
material into two channels.
SUMMARY OF THE INVENTION
The present invention concerns an improved set of design criteria
and their solution to create a decoding matrix having optimum
psychoacoustic performance in reproducing encoded multichannel
material as well as standard two channel material. This decoding
matrix maintains high separation between the left and right
components of stereo signals under all conditions, even when there
is a net forward or rearward bias to the input signals, or when
there is a strong sound component in a particular direction, while
maintaining high separation between the various outputs for signals
with a defined direction, and non-directionally encoded components
at a constant acoustic level regardless of the direction of the
directionally encoded components of the input audio signals. The
decoding matrix includes frequency dependent circuitry that
improves the balance between front and rear signals, provides
smooth sound motion around a seven channel version of the system,
and makes the sound of a five channel version closer to that of a
seven channel version.
Additionally, this invention concerns an improved set of design
criteria and their solution to create an encoding circuit for the
encoding of multi-channel sound into two channels for reproduction
in standard two channel receivers and by matrix decoders.
The present invention is part of a continuing effort to refine the
encoding of multichannel audio signals into two separate channels,
and the separation of the resulting two channels back into the
multichannel signals from which they w ere derived. One of the
goals of this encode/decode process is to recreate the original
signals as perceptually identical to the originals as possible.
Another important goal of the decoder is to extract five or more
separate channels from a two channel source that was not encoded
from a five channel original. The resulting five channel
presentation must be at least as musically tasteful and enjoyable
as the original two channel presentation.
The derivation of suitable variable matrix coefficients and the
variable matrix coefficients themselves have been improved. To
assist the understanding of these improvements, this document makes
reference to U.S.
U.S. Pat. No. 4,862,502 (1989) (referred to in this document as the
"'89 patent");
U.S. Pat. No. 5,136,650 (1992) (referred to in this document as the
"'92 patent"); U.S. patent application Ser. No. 08/684,948, filed
in July 1996 (now issued U.S. Pat. No. 5,796,844 (1998)) (referred
to in this document as the "July '96 application"); and U.S. patent
application Ser. No. 08/742,460 (now issued U.S. Pat. No. 5,870,480
(1999)) (referred to in this document as the "November '96
application"). Commercial versions of the decoder based upon the
November '96 application will be referred to in this document as
"Version 1.11" or "V1.11". Some further improvements were disclosed
in Provisional Patent Application No. 60/058,169, filed September
1997 (referred to in this document as "Version 2.01" "or V2.01."
Further, Versions V1.11 and V2.01, and the decoders presented in
this application will be referred to in this document collectively
as the "Logic 7.RTM. decoders." Additionally, the following are
referenced in this application: [1] "Multichannel Matrix Surround
Decoders for Two-Eared Listeners," David Griesinger, AES preprint
#4402, October, 1996, and [2] "Progress in 5-2-5 Matrix Systems,"
David Griesinger, AES preprint #4625, September, 1997.
An active matrix having certain properties that maximize its
psychoacoustic performance has been realized. Additionally,
frequency dependent modifications of certain outputs of the active
matrix have also been realixed. Further, active circuitry that
encodes five input channels into two output channels is provided
that will perform optimally with the decoders presented in this
application, standard two channel equipment, and industry standard
Dolby.RTM. Pro-Logic.RTM. decoders.
The active matrix decoder has matrix elements that vary depending
on the directional component of the incoming signals. The matrix
elements vary to reduce the loudness of directionally encoded
signals in outputs that are not involved in producing the intended
direction, while enhancing the loudness of these signals in outputs
that are involved in reproducing the intended direction, while at
all times preserving the left/right separation of any
simultaneously occuring input signals. Moreover, these matrix
elements restore the left/right separation of decorrelated two
channel material, which has been directionally encoded, by
increasing or decreasing the blend between the two inputs. For
example, restoration is achieved using stereo width control. In
addition, these matrix elements may be designed to preserve the
energy balance between the various components of the input signal,
as much as possible, so that the balance between vocals and
accompaniment is preserved in the decoder outputs. As a
consequence, these matrix elements preserve both the loudness and
the left/right separation of the non-directionally encoded elements
of the input sound.
Additionally, the decoders may include frequency dependent circuits
that improve the compatibility of the decoder outputs when standard
two channel material is played, that convert the inputs into two
surround outputs (a five channel decoder) or four surround outputs
(a seven channel decoder), and that modify the spectrum of the rear
channels in a five channel decoder so that the sound direction is
perceived to be more like the sound direction produced by a seven
channel decoder.
The encoders mix five (or five full-range plus one low frequency)
input channels into two output channels so that the energy of that
input is preserved in the output when the input level of a
particular input is strong; the direction of a strong input is
encoded in the phase/amplitude ratio of the output signals; the
strong signals can be panned between any two inputs of the encoder,
and the output will be correctly directionally encoded. In
addition, decorrelated material applied to the two rear inputs of
the encoder will be encoded into two output channels so that the
left/right separation of the inputs will be preserved when the
encoder output is decoded by the decoders presented in this
document; in-phase inputs will produce a two channel output that
will be decoded to the rear channels of the decoders presented in
this document and decoders using the Dolby.RTM. standard;
anti-phase inputs will produce outputs that will be decoded as a
non-directional signal when decoded by the decoders presented in
this document or by decoders using the Dolby.RTM. standard; and low
level reverberant signals applied to the two rear inputs of the
encoder will be encoded with a 3 dB level reduction.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features believed characteristic of the encoders and
decoders are set forth in the appended claims. These encoders and
decoders, as well as other features and advantages of the encoders
and decoders, will best be understood by reference to the following
detailed description of an illustrative embodiment when read in
conjunction with the accompanying drawing figures, where:
FIG. 1 is a block diagram of a direction detection section and a
two to five channel matrix section of a decoder;
FIG. 2 is a block diagram of a five-channel frequency-dependent
active signal processor circuit, which may be connected between the
outputs of the matrix section of FIG. 1 and the decoder
outputs;
FIG. 3 is a block diagram of a five-to-seven channel
frequency-dependent active signal processor, which may
alternatively be connected between the outputs of the matrix
section of FIG. 1 and the decoder outputs;
FIG. 4 is a block schematic of an active five-channel to
two-channel encoder;
FIG. 5 is a three-dimensional graph of a Left Front Left (LFL)
matrix element from the '89 patent and Dolby.RTM. Pro-Logic.RTM.
scaled so that the maximum value is one;
FIG. 6 is a three-dimensional graph of a Left Front Right (LFR)
matrix element from the '89 patent and Dolby.RTM. Pro-Logic.RTM.
scaled by 0.71 so that the minimum value is -0.5 and the maximum
value is +0.5;
FIG. 7 is a three-dimensional graph of the square root of the sum
of the squares of LFL and LFR matrix elements from the '89 patent
scaled so that the maximum value is one;
FIG. 8 is a three-dimensional graph of the square root of the sum
of the LFL and LFR matrix elements from the November '96
application No. scaled so that the maximum value is 1;
FIG. 9 is a three-dimensional graph of the LFL matrix element from
V1.11;
FIG. 10 is a three-dimensional graph of a partially completed LFL
matrix element;
FIG. 11 is a graph showing the behavior of the LFL and LFR matrix
elements along the rear boundary between left and full rear;
FIG. 12 is a three-dimensional graph of the fully completed LFL
matrix element as viewed from the left rear;
FIG. 13 is a three-dimensional graph of the fully completed LFR
matrix element;
FIG. 14 is a three-dimensional graph of the root mean squared sum
of the LFL and LFR matrix elements;
FIG. 15 is a three-dimensional graph of the square root of the sum
of the squares of the LFL and LFR matrix elements, including the
correction to the rear level, viewed from the left rear;
FIG. 16 is a graph showing the values of the center matrix elements
that should be used in a Dolby.RTM. Pro-Logic.RTM. decoder as a
function of cs in dB (the solid curve), and the actual values of
the center matrix elements used in the Dolby.RTM. Pro-Logic.RTM.
decoder (the dotted curve);
FIG. 17 is a graph showing the ideal values for the center matrix
elements of the Dolby.RTM. Pro-Logic.RTM. decoder (the solid
curve), and the actual values of the center matrix elements used in
the Dolby.RTM. Pro-Logic.RTM. decoder (the dotted curve);
FIG. 18 is a three-dimensional graph of the square root of the sum
of the squares of the LRL and Left Rear Right (LRR) matrix
elements, using the matrix elements of V1.11;
FIG. 19 is a graph of the numerical solution for GS(lr) and GR(lr)
that result in a constant power level along the cs=0 axis and zero
output along the boundary between left and center;
FIG. 20 is a three-dimensional graph of the square root of the sum
of the squares of LRL and LRR using values for GR and GS determined
according to the present invention;
FIG. 21 is a three-dimensional graph of the Center Left (CL) matrix
element of the four channel decoder in the '89 patent and the
Dolby.RTM. Pro-Logic.RTM. decoder, which can also represent the
Center Right (CR) matrix element with left and right
interchanged;
FIG. 22 is a three-dimensional graph of the Center Left (CL) matrix
element in V1.11;
FIG. 23 is a graph showing the center output channel attenuation
needed for the new LFL and LFR matrix elements (the solid curve),
and the center attenuation for a standard Dolby.RTM. Pro-Logic.RTM.
decoder (the dotted curve);
FIG. 24 is a graph showing the ideal center attenuation for the
"film" strategy (the solid curve), another center attenuation for
the "film" strategy(the dashed curve), and the center attenuation
for the standard Dolby.RTM. decoder (the dotted curve);
FIG. 25 shows the center attenuation used for the "music"
strategy;
FIG. 26 is a graph showing the value of GF needed for constant
energy ratios with the "music" center attenuation GC (the solid
curve), the previous value of the LFR matrix element sin(cs)*corrl
(the dashed curve), and the value of sin(cs) (the dotted
curve);
FIG. 27 is a three-dimensional graph of the LFR matrix element,
including the correction for center level along the lr=0 axis;
FIG. 28 is a three-dimensional graph of the CL matrix element with
the new center boost function; and
FIG. 29 is a graph of the output level from the left front output
(the dotted curve) and the center output (the solid curve) as a
strong signal pans from center to left.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
1. General Description of the Decoder
The decoder will be described in terms of two separate parts.
The first part is a matrix that splits two input channels into five
output channels (the input channels are usually identified as
center, left front, right front, left rear, and right rear). The
second part consists of a series of delays and filters that modify
the spectrum and the levels of the two rear outputs. One of the
functions of the second part is to derive an additional pair of
outputs, a left side and a right side, to produce a seven channel
version of the decoder. In contrast, the two additional outputs
described in the November '96 application were derived from an
additional pair of matrix elements, which were included in the
original matrix.
In the mathematical equations describing the decoder and encoder
the standard typographical conventions will be used for most
variables. Simple variables will be in italic type, vector
quantities will be in bold lower case type, and matrixes will be in
bold upper case type. Matrix elements that are coefficients from a
named output channel resulting from a named input channel will be
in normal upper case type. Some simple variables such as lr and cs
will be indicated by two-letter names that do not represent the
product of two separate simple variables. Other variables, such as
l/r and c/s, represent the values of left-right and center-surround
ratios in terms of control signal voltages derived from these
ratios. These conventions have also been used in the patents and
patent applications cited in this document. Program segments in the
Matlab language will also be distinguished by the use of indented
lines. Equations will be numbered to distinguish them from Matlab
assignment statements, and to provide a reference for specific
features.
FIG. 1 is a block diagram of the first part of the decoder, which
is a two channel to five channel matrix 90. The left half of FIG.
1, partitioned by a vertical dashed line, shows a circuit for
deriving the two steering voltages l/r and c/s. These steering
voltages represent the degree to which the input signals have an
inherent or encoded directional component in the left/right or
front/back directions, respectively. This part of FIG. 1 will not
be explicitly discussed in this application, because it has been
fully described in the patent and patent applications cited in this
document, which are incorporated by reference.
In FIG. 1 the directional detection circuit of decoder 90
comprising elements 92 through 138 is followed by a 5.times.2
matrix (shown to the right of the vertical dashed line). The
elements of this matrix, 140 through 158, determine the amount of
each input channel linearly combined with another input channel to
form each output channel. These matrix elements are assumed to be
real (the case of complex matrix elements is described in the
November '96 application). The matrix elements are functions of the
two steering voltages l/r and c/s, mathematical formulae for which
are presented in the November '96 application. Improvements have
been made to these formulae.
2. A Brief Description of the Steering Voltages
As shown in FIG. 1, the steering voltages c/s and l/r are derived
from the logarithm of the ratio of the left input amplitude at
terminal 92 to the right input amplitude at terminal 94, and the
logarithm of the ratio of the sum amplitude (the sum of the left
input amplitude and the right input amplitude) to the difference
amplitude (the difference between the left input amplitude and the
right input amplitude). In V1.11 and V2.01, the unit of the
steering voltages is decibels. However, when describing the matrix
elements, it is convenient to express l/r and c/s as angles that
vary from +45 degrees to -45 degrees. The steering voltages l/r and
c/s can be converted into angles lr and cs, respectively, according
to the following equations:
The angles lr and cs determine the degree to which the input
signals have a directional component. For example, when the inputs
to the decoder are decorrelated, both lr and cs are zero. For a
signal that comes from the center only, lr is zero, and cs is 45
degrees. For a signal that comes from the rear, lr is zero, and cs
is -45 degrees. Similarly, for a signal that comes from the left,
lr is 45 degrees and cs is zero, and for a signal that comes from
the right, lr is -45 degrees, and cs is zero. It may be assumed
that the input was encoded so that lr=22.5 degrees and cs=-22.5
degrees for left rear signals, and lr=22.5 degrees, and cs=-22.5
degrees for right rear signals.
Due to the definitions of l/r and c/s and the derivation of lr and
cs, the sum of the absolute value of lr and cs cannot be greater
than 45 degrees. Therefore, the allowed values of lr and cs form a
surface bounded by the locus of abs(lr)-abs(cs)=45 degrees. Any
input signal that produces values of lr and cs that lie along the
boundary of this surface is fully localized, which means that the
input signal consists of a single sound that has been encoded to
come from a particular direction.
In this application extensive use will be made of graphs depicting
the matrix elements as functions over this two dimensional surface.
In general, the derivation of the matrix elements will be different
in the four quadrants of this surface. In other words, the matrix
elements are described differently depending on whether the
steering is to the front or to the rear, and whether the steering
is to the left or the right. Considerable work is devoted to
insuring that the surface is continuous across the boundaries
between quadrants, thus addressing the occasional lack of
continuity experienced by V1.11.
3. Frequency Dependent Elements
The matrix elements shown in FIG. 1 are real and thus frequency
independent. All signals in the inputs will be directed to the
outputs depending on the derived angles lr and cs. Additionally,
low frequencies and very high frequencies may be attenuated in the
derivation of lr and cs from the input signals by filters not shown
in FIG. 1. However, the matrix itself is broadband.
There are several advantages to applying frequency dependent
circuits to the signals after the matrix. One of these frequency
dependent circuits, the phase shift network 170 at the right side
output 180 in FIG. 1, is described in the November '96 application.
A five channel version of the additional frequency dependent
circuits is shown in FIG. 2. These circuits do not have fixed
parameters and the frequency and level behavior is dependent on the
steering angles lr and cs. The frequency dependent circuits
accomplish several purposes. First, in both a five channel and a
seven channel decoder, the additional elements allow the apparent
loudness of the rear channels to be adjusted when the steering is
neutral (lr and cs 0) or toward the front (cs>0). In the
November '96 application, this attenuation was performed as part of
the matrix itself and was frequency independent. It has been found
through theoretical studies and listening tests that it is highly
desirable for the low frequencies to be reproduced from the sides
of the listener. Thus, in the decoder presented here, only the high
frequencies are attenuated by variable low pass filters 182, 184,
188, and 190.
The high frequencies are attenuated in the rear channels when the
steering is nearly always neutral or forward. Elements 188 and 190
attenuate the frequencies above 500 Hz and elements 182 and 184
attenuate the frequencies above 4 kHz using a background control
signal 186 (to be defined later). The occasional presence of sounds
that are steered rearwards reduces the attenuation, which is a
feature that automatically distinguishes surround encoded material
from ordinary two channel material.
Elements 192 and 194, in the five channel version modify the
spectrum of the sound when the steering is toward the rear
(cs<0) using the c/s signal 196, such that the loudspeakers are
perceived as being located behind the listener even if the actual
position of the loudspeakers is to the side. The modified left
surround and right surround signals appear at terminals 198 and
200, respectively. Additional details of this circuit will be
presented in a later section.
FIG. 3 shows the seven channel version of the frequency dependent
elements. As before the first set of filters 182, 184, 188, and
190, attenuate the upper frequencies of the side and rear outputs
when the steering is neutral or forward, and are controlled by the
background control signal 186. This attenuation also results in a
more forward sound image, and can be adjusted to the listener's
taste. As the steering represented by the c/s signal 196 moves to
the rear, additional circuits 202, 204, 206, and 208, act to
differentiate the side outputs from the rear outputs. As steering
moves rearward, the attenuation in the side speakers is removed by
elements 204 and 206 to produce a side oriented sound. As steering
moves further to the rear, the attenuation of elements 204 and 206
is reinstated and increased. This causes the sound to move smoothly
from the front loudspeakers to the side loudspeaker(s) and then to
the rear loudspeakers. However, the sound in the rear loudspeakers
has a delay of about 10 ms, which is produced by the delay elements
202, and 208. Because the low frequencies are not affected by these
circuits, the low frequency loudness in the side speakers (which is
responsible for the perception of spaciousness) is not affected by
the motion of the sound.
4. General Description of the Encoder
FIG. 4 shows a block diagram of an encoder designed to
automatically mix five input channels into two output channels. The
architecture is quite different from the encoder described in the
November '96 application. An object of the encoder in FIG. 4 (the
"new encoder") is to preserve the musical balance of the five
channel original in the two output channels, while providing
phase/amplitude cues that allow the original five channels to be
extracted from the two output channels by a decoder. The new
encoder includes active elements that ensure that the musical
balance is preserved. Another object of the new encoder is to
automatically create a two channel mix from a five channel
recording, that can be reproduced by an ordinary two channel system
with the same artistic quality as the five channel original.
Unlike the encoder of the November '96 application, the new encoder
allows input signals to be panned between any of the five inputs of
the encoder. For example, a sound may be panned from the left front
input to the right rear input. When the resulting two channel
signal is decoded by the decoder described in this application, the
result will be quite close to the original sound. Decoding through
an earlier surround decoder will also be similar to the
original.
5. Design Goals for the Decoder Active Matrix Elements
The goals of the current decoder--include: having variable matrix
values that reduce directionally encoded audio components in
outputs that are not directly involved in reproducing them in the
intended direction; enhancing directionally encoded audio
components in the outputs that are directly involved in reproducing
them in the intended direction to maintain constant total power for
such signals; preserving high separation between the left and right
channel components of non-directional signals, regardless of the
steering signals; and maintaining the loudness (defined as the
total audio power level of non-directional signals) at an
effectively constant level, whether directionally encoded signals
are present and regardless of their intended direction.
Most of these goals are ostensibly shared by all matrix decoders.
One of the most important goals is explicitly maintaining high
separation between the left and right channels of the decoder under
all conditions. All previous four channel decoders are unable to
maintain separation in the rear because they provide only a single
rear channel. Five other channel decoders can maintain separation
in many ways. The decoder described in this application meets this
goal in a manner similar to that used by V1.11, and meets
additional goals as well.
The November '96 application also describes many smaller
improvements to a decoder, such as circuits to improve the steering
signals accuracy, and a variable phase shift network to switch the
phase shift of one of the rear channels during strong rear
steering. These features (included in V1.11) are retained in the
current decoder.
In FIG. 4 the front input signals L, C and R are applied to input
terminals 50, 52, and 54 respectively. L and R go directly to
adders 278 and 282 respectively, while C is attenuated by a factor
.function.cn in attenuator 372 before being applied to adders 278
and 282. A gain of 2.0 is applied to the low frequency effects
signal LFE by element 374 before LFE is applied to adders 278 and
282.
The surround input signals LS and RS are applied to input terminals
62 and 64, respectively. The LS signal passes through attenuator
378, which has gain .function.s(l,ls), and the RS signal passes
through attenuator 380, which has gain .function.s(r,rs). The
outputs of these attenuators 378 and 380 are passed into
cross-coupling elements 384 and 386, respectively, each having a
gain factor of -crx, where crx is nominally 0.383. The
cross-coupled signals from cross-coupled elements 386 and 384 are
fed to summers 392 and 394, respectively, which also receive the
attenuated LS and RS signals, respectively, from 0.91 attenuators
388 and 392, respectively. The outputs of summers 392 and 394, are
applied to inputs of the adders 278 and 282, respectively. This
positions the side elements at 45 degrees left and right,
respectively, of center rear in the decoded space.
LS and RS also pass through attenuator 376, which has gain
.function.c(l,ls), and attenuator 382, which has gain
.function.c(r,rs), respectively, and then through a similar
arrangement of cross-coupling elements 396, 398, 402, 404, 406, and
408. The summers 406 and 408 have outputs that position the left
rear and right rear inputs at 45 degrees left and right,
respectively, of center rear, as before. However, LS and RS also
pass through phase shifter elements 234 and 246, respectively,
while the left and right signals from adders 278 and 282,
respectively, pass through phase shifter elements 286 and 288,
respectively. Each of these phase shifter elements is an all-pass
filter, where the phase response for elements 286 and 288 is
.phi.(.function.), and for elements 234 and 246 is
.phi.(.function.)-90.degree.. Calculation of the component values
required in these filters is well known in the art. The phase
shifter elements cause the outputs of summers 406 and 408 to lag
the outputs of adders 278 and 282 by 90 degrees at all frequencies.
The outputs of all-pass filters 234 and 286 are combined by summer
276 to produce the A (or left) output signal at terminal 44, while
the outputs of all-pass filters 246 and 288 are combined by summer
280 to produce the B (or right) output signal at terminal 46.
The gain functions .function.s and .function.c are designed to
allow strong surround signals to be presented in phase with the
other sounds while weak surround signals pass through the 90 degree
phase-shifted path to retain constant power for decorrelated
"music" signals. The value of crx can also change and varies the
angle from which the surround signals are heard.
6. Design improvements since the November '96 application
One of the most noticeable improvements made to the decoder and
encoder of the November '96 application is the change in the center
matrix elements and the left and right front matrix elements when a
signal is steered in the center direction. There were two problems
with the center channel as previously encoded and decoded. The most
obvious problem was that, in a five channel matrix system, the use
of a center channel was inherently in conflict with the goal of
maintaining as much left/right separation as possible. If the
matrix is to produce a sensible output from conventional two
channel stereo material when the two input channels have no
left/right component, the center channel must be driven with the
sum of the left and right input channels. Thus both the left
decoder input and the right decoder input will be reproduced by the
center speaker and sounds that were originally only in the left or
right channel will also be reproduced from the center. This results
in the apparent position of these sounds being drawn to the middle
of the room. The degree to which this occurs depends on the
loudness of the center channel.
The '89 patent and the '92 patent used center matrix elements that
had a minimum value of 3 dB compared to the left and right
channels. When the inputs to the decoder were decorrelated, the
loudness of the center channel was equal to the loudness of the
left and right channels. As steering moved forward, the center
matrix elements increased another 3 dB, which strongly reduced the
width of the front image. Instruments that should have sounded as
if positioned to either the left or the right of the sound image
are always drawn toward the center of the sound image.
The November '96 application used center matrix elements that had a
minimum value 4.5 dB less than values previously used. This minimum
value was chosen on the basis of listening tests and caused a
pleasing spread to the front image when the input material was
uncorrelated (which is the case with orchestral music). Therefore,
the front image was not seriously narrowed. However, as the
steering moved forward, these matrix elements were increased and
ultimately reach the values used in the Dolby.RTM. matrix.
Experience with V1.11 showed that although the reduction in center
channel loudness solved the spatial problem, the power balance in
the input signals was not preserved through the matrix.
Mathematical analysis revealed that not only was V1.11 in error
with regard to the power balance, but the Dolby.RTM. decoder and
other previous decoders were also in error. Paradoxically, although
the center channel was too strong from the standpoint of
reproducing the width of the front image, it was too weak to
preserve power balance. The problem was particularly severe for the
standard Dolby.RTM. decoder (the decoder of Mandel). In the
standard Dolby.RTM. decoder, the rear channels are stronger than in
the decoder of the '89 patent. As a result, the center channel must
be stronger to preserve the power balance. The lack of power
balance in the center channel has been a continual problem for the
Dolby.RTM. decoder. In fact, Dolby.RTM. recommends that the sound
mix engineer always listen to the balance through the matrix, so
compensation can be made during the mixing process for the lack of
power balance in the matrix during the mixing process.
Unfortunately, modern films are mixed for five-channel release, and
automatic encoding to two channels can lead to problems with the
dialog level.
Additional analysis and listening tests showed that films and music
require different solutions to the balance problem. For films, it
is most useful to preserve the left and right front matrix elements
from the November '96 application. These elements eliminate the
center channel information from the left and right front channels
as much as possible, which minimizes dialog leakage into the front
left and right channels. In a new "film" design, the power balance
is corrected by changing the center matrix elements so that the
center channel loudness increases more rapidly than in the standard
decoder as the steering moves forward (as cs becomes greater than
zero.) In practice it is not necessary for the final value of the
center matrix elements to be higher than those in the standard
decoder, because this condition is reached when only the center
channel is active. It is only necessary for the center channel
level to be stronger than the standard decoder when there are
approximately equal levels in the center, left and right
channels.
In the "film" strategy, the center channel loudness is increased to
preserve the power balance in the input signals, while minimizing
the center channel component in all the other outputs. This
strategy seems to be ideal for films, where the major use of the
center channel is for dialog, and dialog from positions other than
the center is not expected. The major disadvantage of this strategy
is that anytime there is significant center steering, such as that
which occurs in many types of popular music, the front image is
narrowed. However, the advantages for film, which include minimum
dialog leakage into the front channels and excellent power balance,
outweigh this disadvantage.
For music another strategy is adopted, in which the center channel
loudness is permitted to increase at the same rate described in the
November '96 application, up to a middle value of the steering
(where cs>22.5 degrees). To restore the musical balance, the
left and right front matrix elements are altered so that the center
component of the input signals is not entirely removed. The amount
of the center channel component in the left and right front
channels is adjusted so that the sound power from all the outputs
of the decoder matches the sound power in the input signals,
without excessive loudness in the center.
In this strategy, all three front speakers reproduce center channel
information present in the original encoded material. The most
useful version of this strategy limits the steering action when the
center component of the input is 6 dB stronger in the center output
than in either of the two other front outputs. This is done by
simply limiting the positive value of cs.
This new strategy, which allows the center channel component to
come from all three front speakers, and limits the steering action
when the center is 6 dB louder than the front left and right, is
excellent for all types of music. Encoded five-channel mixes and
ordinary two-channel mixes are decoded with a stable center and
adequate separation between the center channel and the left and
right channels. Note that unlike previous decoders, the separation
between center and left and right is deliberately not complete. A
signal intended to come from the left is eliminated from the center
channel, but not the other way around. For music, the high lateral
separation and stable front image that this strategy offers
outweighs this lack of complete separation. Listening tests using
this setting on films reveal that although there was some dialog
coming from the left and right front speakers, the stability of the
resulting sound image was quite good. The resulting sound was
pleasant and not distracting. Therefore, hearing a film with the
decoder set for music does not detract from the artistic quality of
the film. However, listening to a music recording with the decoder
set for film is more problematic.
Possibly the next most obvious improvement made to the decoder and
encoder of the November '96 application is the increase in
separation between the front channels and the rear channels when a
signal is steered to the left front or the left rear directions.
V1.11 used the matrix elements of the '89 patent for the front
channels under these conditions. These matrix elements did not
fully eliminate a rear steered signal unless it was steered to the
full rear position (which is the position half way between left
rear and right rear). When steering was to left rear or right rear
(not full rear), the left or right front output had an output that
was 9 dB less than the corresponding rear output. In the present
decoder the front matrix elements are modified to eliminate sound
from the front when steering is anywhere between left rear and
right rear.
7. Improvements to the Rear Matrix Elements
The improvements to the rear matrix elements are not immediately
obvious to a typical listener. These improvements correct various
errors in the continuity of the matrix elements across the
boundaries between quadrants. They also improve the power balance
between steered signals and unsteered signals under various
conditions. A mathematical description of the matrix elements that
includes these improvements will be given later in this
document.
8. Detailed Description of the Active Matrix Elements
The Matlab Language
The math used to describe the matrix elements is not based on
continuous functions of the variables cs and lr. In general there
are conditionals, absolute values, and other non-linear
modifications to the formulae. For this reason the matrix elements
will be described using a programming language. The Matlab language
provides a simple method of checking the formulation graphically.
Matlab is very similar to Fortran or C. The major difference is
that variables in Matlab can be vectors which means that each
variable can represent an array of numbers in sequence. For
example, the variable x can be defined according to an expression
"x=1:10." Defining x in this manner in Matlab creates a string of
ten numbers with the values of one to ten. The variable x includes
all ten values and is described as a vector (which is a 1 by 10
matrix). An individual number within each vector can be accessed or
manipulated. For example, the expression "x(4)=4" will set the
fourth member of the vector x equal to 4. Avariable can also
represent a two dimensional matrix and individual elements in the
matrix can be assigned in a similar way. For example, the
expression "X(2,3)=10" will assign the value 10 to the matrix
element in the second row and third column of the matrix X.
9. Matrix Decoders in Equations and Graphics
Reference [1] presented the design of a matrix decoder that can be
described by the elements of a n.times.2 matrix, where n is the
number of output channels. Each output can be seen as a linear
combination of the two inputs, where the coefficients of the linear
combination are given by the elements in the matrix. In this
document the elements are identified by a simple combination of
letters. Reference [1] described a five-channel and a seven-channel
decoder. Because the conversion from five channels to seven
channels can now be done in the frequency dependent part of the
decoder, what follows is description of a five-channel decoder
only.
Due to from symmetry the behavior of only six elements (such as the
left elements) need to be described. These six elements include the
center elements, the two left front elements, and the two left rear
elements. The right elements can found from the left elements by
simply switching the identity of left and right. The left elements
are indicated by the following notation: CL: The matrix element for
the Left input channel to the Center output channel. CR: The matrix
element for the Right input channel to the Center output channel.
LFL: The Left input channel to the Left Front output channel. LFR:
The Right input channel to the Left Front output channel. LRL: The
Left input channel to the Left Rear output channel. LRR: The Right
input channel to the Left Rear output channel.
These elements are not constant. Their value varies as a two
dimensional function of the apparent direction of the input sounds.
Most phase/amplitude decoders determine the apparent direction of
the input by comparing the ratio of the amplitudes of the input
signals. For example, the degree of steering in the right/left
direction is determined from the ratio of the left input channel
amplitude to the right input channel amplitude. In a similar way,
the degree of steering in the front/back direction is determined
from the ratio of the amplitudes of the sum and the difference of
the input channels.
In this document, the apparent directions of the input signals will
be represented as angles, including one angle for the left/right
direction (lr), and one for the front/back (also known as the
center/surround) direction (cs). The two steering directions lr and
cs are signed variables. When the two input channels are
uncorrelated, both lr and cs are zero and the input signals are,
therefore, unsteered. When the input consists of a single signal
which has been directionally encoded, the two steering directions
have their maximum value however, they are not independent. The
advantage to representing the steering values as angles is that
when there is only a single signal, the sum of the absolute value
of each of the two steering values must equal 45 degrees. When the
input includes some decorrelated material along with a strongly
steered signal, the sum of the absolute values of each of the
steering values must be less than 45 degrees as indicated by the
following equation:
If the values of the matrix elements are plotted over a
two-dimensional plane formed by the steering values, the center of
the plane will have the value (0, 0) and the valid values for the
sum of the absolute values of the steering values will not exceed
45. In practice, it is possible for the sum to exceed 45, due to
the behavior of non-linear filters. To prevent this, a circuit that
limits the lessor of lr or cs so their sum does not exceed 45
degrees may be used, such as the circuit described in the November
'96 application. When the matrix elements are graphed the values
will arbitrarily be set to zero when the valid sum of the input
variables is exceeded. This allows the behavior of the element
along the boundary trajectory (the trajectory followed by a
strongly steered signal) to be viewed directly. The graphics were
created using Matlab. In the Matlab language, the unsteered
position is (46, 46) because Matlab requires the angle variable to
be 1 more than the actual angle value.
Previous designs for matrix decoders tended to consider only the
behavior of the matrix in response to a strongly steered signal,
which is the behavior of the matrix elements a round the boundary
of the surface formed by plotting the matrix elements over a
two-dimensional plane defined by the steering values. This is a
fundamental error in outlook because, in real signals (for example,
those found in either film or music), the boundary of the surface
is very seldom reached. For the most part, signals wobble around
the middle of the plane, which is slightly forward of the center.
The behavior of the matrix under these conditions is of vital
importance to the sound. When the elements described in this
document are compared to previous elements, a striking increase in
the complexity of the surface in the middle regions can be seen. It
is this complexity which is responsible for the improvement in the
sound.
However, such complexity has a price. The elements described in
this document are designed to be almost entirely described by
one-dimensional lookup tables, which are trivial in a digital
implementation. However, unlike the matrix of the '89 patent,
designing an analog version with similar performance is not
trivial.
In the sections that follow, several different versions of the
matrix elements are contrasted. The earliest are elements from the
'89 patent. These elements are identical to the elements of a
standard (Dolby) surround processor in the left, center, and right
channels, but not in the surround channels. In the design of the
'89 patent, the surround channel is treated symmetrically to the
center channel. In the standard (Dolby.RTM.) decoder, the surround
channel is treated differently.
The elements presented are not always correctly scaled. In general
they are presented so that the unsteered value of the non-zero
matrix elements for any given channel is one. In practice, the
elements are usually scaled so that the maximum value of each
element is one or lower. In any case, the scaling of the elements
is additionally varied in the calibration procedure. It may be
assumed that the matrix elements presented in this document are
scalable by the appropriate constants.
10. The Left Front Matrix Elements in Our '89 Patent
Assume that cs and lr are the steering directions in degrees in the
center/surround and left/right axis respectively. In the '89
patent, the equations for the front matrix elements are defined
according to equations (3a), (3b), (3c), (3d), (3e), (3f), (3g),
and (3h). In the left front quadrant:
In the right front quadrant:
In the left rear quadrant (cs is negative):
LFL=1-0.5*G(cs)+0.41*G(lr) (3e)
In the right rear quadrant:
The function G(x) was determined experimentally in the '89 patent
and was specified mathematically in the '92 patent. G(x) varies
from 0 to 1 as x varies from 0 to 45 degrees. When steering is in
the left front quadrant (lr and cs are both positive), G(x) is
equal to 1-.vertline.r.vertline./.vertline.l.vertline. where
.vertline.r.vertline. and .vertline.l.vertline. are the right and
left input amplitudes. G(x) can also be described in terms of the
steering angles using various formulae. One of these is given in
the '92 patent, and another will be given later in this document.
Graphical representations of the LFL and LFR matrix elements
plotted three dimensionally against the lr and cs axes are shown in
FIG. 5 and FIG. 6.
In reference [1], these elements were improved by adding a
requirement that the loudness of unsteered material should be
constant regardless of the direction of the steering.
Mathematically this means that the root mean square sum of the LFL
and LFR matrix elements should be a constant. This goal should be
altered in the direction of the steering, which means that when the
steering is full left, the sum of the squares of these matrix
elements should rise by 3 dB. FIG. 7 shows the sum of the squares
of these elements and demonstrates that the above matrix elements
do not meet the requirement of constant loudness. In FIG. 7, the
value is constant at 0.71 along the axis from unsteered to right.
The value along the axis from unsteered to left rises 3 dB to one,
and the value along the axis from unsteered to center or from
unsteered to rear falls 3 dB to 0.5. The value along the axis from
unsteered to rear is hidden by the peak at left. The rear direction
level is identical to that at the center direction.
In the November '96 application and Reference [1], the amplitude
errors in FIG. 7 were corrected by replacing the function G(x) in
the matrix equations with sines and cosines: FIG. 8 shows a graph
of the sum of the squares of the corrected elements LFL and LFR,
which are described by the equations (4a)-(4h) below. Note the
constant value of 0.71 in the entire right half of the plane, and
the gentle rise to one toward the left vertex. For the left front
quadrant:
For the right front quadrant:
For the left rear quadrant:
For the right rear quadrant:
LFL=cos(-cs) (4g)
11. Improvements to the Left Front Matrix Elements
To improve the performance of the matrix elements with stereo music
that was panned forward and to increase the separation between the
front channels and the the rear channels when stereo music was
panned to the rear, an additional boost along the cs axis was added
in the front, and a cut along the cs axis was added in the rear,
respectively (the "March '97 version"). However, the basic
functional dependence among these matrix elements was maintained.
For the front left quadrant:
For the right front quadrant:
For the left rear quadrant:
For the right rear quadrant:
where the function G(x) is the same as the one in the '89 patent.
When expressed with angles as an input, G(x) is equal to:
FIG. 9 shows a plot of LFL resulting from equations (5a)-(5h). Note
that as the steering moves toward center, the boost is applied both
along the lr=0 axis, and along the left to center boundary. Note
also the reduction in level as the steering moves to the rear.
The performance of the March '97 circuit can be improved. The first
problem with the March '97 version is in the behavior of the
steering along the boundaries between left and center, and between
right and center. As shown in FIG. 9, the value of the LFL matrix
element increases to a maximum half-way between left and center as
a strong single signal pans from the left to the center. This
increase is an unintended consequence of the deliberate increase in
level for the left and right main outputs as a center signal is
added to stereo music.
When a stereo signal is panned forward, it is desirable for the
levels of the left and right front outputs to rise to compensate
for the removal of the correlated component from these outputs by
the matrix. However, this level increase should only occur when the
lr component of the inputs is minimal (when there is no net left or
right steering). Therefore, the boost is only needed along the lr=0
axis. When lr is non-zero, the matrix element should not be
boosted.
The increase implemented in the March of '97 circuit was
independent of lr, and therefore resulted in a level increase when
a strong signal was panned across the boundary. This problem can be
solved by using an additive term to the matrix elements, instead of
a multiply. A new steering index (the boundary limited cs value) is
defined with the following Matlab code:
Assume both lr and cs>0--we are in the left front quadrant
(assume cs and lr follow the Matlab conventions of varying from 1
to 46) % find the bounded c/s if (cs<24) bcs=cs-(lr-1); if
(bcs<1) % this limits the maximum value bcs=1; end else
bcs=47-cs-(lr-1); if (bcs<1) bcs=1; end end
If cs<22.5 and lr=0, (in the Matlab convention cs<24 and lr
1) bcs is equal to cs. However, bcs will decrease to zero as lr
increases. If cs>22.5, bcs also decreases as lr increases.
To find the correction function needed, the difference between the
boosted matrix elements and the non-boosted matrix elements are
found along the lr=0 axis. This difference is called cos_tbl_plus
and sin_tbl_plus. Using Matlab code: a=0:45; % define a vector in
one degree steps. a has the values of 0 to 45 degrees
a1=2*pi*a/360: % convert to radians % now define the sine and
cosine tables, as well as the boost tables for the front
sin_tbl=sin(a+1); cos_tbl=cos(a+1);
cos_tbl_plus=cos(a1).*corrl(a+1); cos_tbl_plus=cos
tbl_plus-cos_tbl; % this is the one we use
cos_tbl_minus=cos(a1)./corr(a+1); sin_tbl_plus=sin(a1).*corrl(a+1);
sin_tbl_plus=sin tbl_plus-sin_tbl; % this is the one we use
sin_tbl_minus=sin(a1)./corr(a+1);
The vectors sin_tbl_plus and cos_tbl_plus are the difference
between a plain sine and cosine, and the boosted sine and cosine.
LFL and LFR are defined according to the following equations:
In the front right quadrant LFL and LFR are similar, but do not
include the +0.41*G term. These new definitions lead to the matrix
element shown graphically in FIG. 10. In FIG. 10, the new element
has the correct amplitude along the left to center boundary, as
well as along the center to right boundary.
The steering in the rear quadrant is not optimal either. When the
steering is toward the rear, the above matrix elements are given
by:
These matrix elements are very nearly identical to the elements in
the '89 patent. Consider the case when a strong signal pans from
left to rear. The elements in the '89 patent were designed so that
there was a complete cancellation of the output from the front left
output only when this signal is fully to the rear (cs=-45. lr=0).
However, it is desirable for the the left front output to be zero
when the encoded signal reaches the left rear direction (cs=-22.5
and lr=22.5), and for=the left front output to remain at zero as
the signal pans further to full rear. The matrix elements used in
March '97 circuit result in the output in the front left channel
being about -9 dB when a signal is panned to the left rear
position. This level difference is sufficient for good performance
of the matrix, but it is not as good as it could be.
Performance can be improved by altering the LFL and LFR matrix
elements in the left rear quadrant. The concern here is how the
matrix elements vary along the boundary between left and rear. The
mathematical method given in reference [1] can be used to find the
behavior of the elements along the boundary. If it is assumed that
the amplitude of the left front output should decrease with the
function F(t) as t varies from 0 degrees (left) to -22.5 degrees
(left rear), the matrix elements are defined according to the
following equations:
If F(t)=cos(4*t) and the correct sign is choosen, equations (9a)
and (9b) simplify to the following equations:
LFL=cos(t)*cos(4*t)+sin(t)*sin(4*t) (9c)
A plot of these coefficients is shown in FIG. 11, where LFL (solid
curve) and LFR (dotted curve) are plotted as a function of t.
Because all angles in Matlab are integers, the slight glitch in the
middle is due to the absence of a point at 22.5 degrees.
These elements work well. As shown in FIG. 11, the front left
output is reduced smoothly to zero as t varies from 0 to 22.5
degrees. However, it is desirable for the output to remain at zero
as the steering continues from 22.5 degrees to 45 degrees (full
rear.) Along this part of the boundary, LFL and LFR are defined
according to the following equations:
These matrix elements are a far cry from the matrix elements along
the lr=0 boundary where, in reference [1], the values were defined
according to the following equations:
These matrix elements are designed to behave properly with a
strongly steered signal (where both cs and lr have maximum values).
The previous matrix elements were successful for signals where lr
is near zero (stereo signals that have been panned to the rear).
Therefore, a method of smoothly transforming the earlier matrix
elements into the newer matrix elements as lr and cs approach the
boundary is needed. One may include approach linear interpolation.
Another approach, which is particularly useful where multiplies are
expensive, includes defining the minimum of lr and cs as a new
variable. One example of this approach is shown in the Matlab
segment below: % new--find the boundary parameter bp=x; if
(bp>y) bp=y; end
and a new correction function which depends on bp: for x=1:24
ax=2*pi*(46-x), 360;
front_boundary_tbl(x)=(cos(ax)-sin(ax))/(cos(ax)+sin(ax)); end for
x=25:46 ax=2*pi*(x-1)/360; front_boundary
tbl(x)=(cos(ax)-sin(ax))/(cos(ax)+sin(ax)); end
LFL and LFR are then defined in this quadrant according to the
following equations:
LFL=cos(cs)/(cos(cs)+sin(cs))-front_boundary.sub.--
tbl(bp)+0.41*G(lr) (11a)
Note the correction of cos(cs)+sin(cs). When cos(cs) is divided by
this factor, the function 1-0.5*G(cs) is obtained, which is the
same as the Dolby.RTM. matrix in this quadrant. Then sin(cs) is
divided by this factor and the earlier function +0.5*G(cs) is
obtained.
Similarly in the right rear quadrant, LFL and LFR are defined
according to the following equations:
A graphical display of LFL and LFR is shown in FIG. 12 and FIG. 13,
respectively.
In FIG. 12, which presents the left rear of the coefficient graph,
there is a large correction along the left-rear boundary. This
large correction causes the front left output to go to zero when
steering goes from left to left rear. The output remains zero as
the steering progresses to full rear. The function is identical to
the Dolby.RTM. matrix along the lr=0 axis and in the right rear
quadrant.
In FIG. 13 there is a large peak in the left to rear boundary. This
works in conjunction with the LFL matrix element to keep the front
output at zero along this boundary as steering goes from left rear
to full rear. Once again, the element is identical to the
Dolby.RTM. matrix in the rear direction along the lr=0 axis and the
rear right quadrant.
One of the major design goals for the matrix is that in any given
output, the loudness of unsteered material presented to the inputs
of the decoder should be constant, regardless of the direction of a
steered signal present at the same time. As explained previously,
this means that the sum of the squares of the matrix elements for
each output should be one, regardless of the steering direction.
However, as explained before, this requirement must be altered when
there is strong steering in the direction of the output in
question. That is, if with regard to the left front output, the sum
of the squares of the matrix elements must increase by 3 dB when
the steering goes full left. The above elements also alter the
requirement somewhat when the steering moves forward and backward
along the lr=0 axis.
FIGS. 14 and FIG. 15 show plots of the square root of the sum of
the squares of the matrix elements for the revised design. In FIG.
14, the 1/(sin(cs)+cos(cs)) correction in the rear quadrant was
deleted so that the accuracy of the resulting sum could be better
visualized. In FIG. 15, there is a 3 dB peak in the left direction,
and a somewhat lesser peak as a signal goes from unsteered to 22.5
degrees in the center direction. This peak is a result of the
deliberate boost of the left and right outputs during half-front
steering. Note that in the other quadrants the rms sum is very
close to one, which was the intent of the design. Because the
method used to produce the elements was an approximation, the value
in the rear left quadrant is not quite equal to one. However, it is
a pretty good match.
In FIG. 15, the unsteered (middle) to right axis has the value one,
the center vertex has the value 0.71, the rear vertex has the value
0.5, and the left vertex has the value 1.41. Note that there is a
peak along the middle to center axis.
12. Rear Matrix Elements During Front Steering
The rear matrix elements in the '89 patent, to which a scaling by
0.71 has been introduced to show the effect of the standard
calibration procedure, are defined according to equations (13a),
(13b), (13a) and (13c). For the front left quadrant:
For the rear left quadrant:
(the right half of the plane is identical but switches LRL and
LRR.)
After a similar calibration, the rear matrix elements in the
Dolby.RTM. Pro-Logic.RTM. are defined according to equations (14a),
(14b), (14c), and (14d). For the front left quadrant:
For the rear left:
The right half of the plane is identical, but switches LRL and LRR.
Note that the Dolby elements and the elements of the '89 patent are
calibrated to be equal in the rear left quadrant when cs=-45
degrees.
13. A Brief Digression on the Surround Level in Dolby.RTM.
Pro-Logic.RTM.
The Dolby.RTM. elements are similar to the elements given in the
'89 patent, except that the boost is not dependent on cs in the
rear. This difference is quite important, because after the
standard calibration procedure, the elements have quite different
values for unsteered signals. In general, the description in this
document of the matrix elements does not consider the calibration
procedure for these decoders and all the matrix elements are
derived with a relatively arbitrary scaling. In most cases, the
elements are presented as if they had a maximum value of 1.41. In
fact, for technical reasons, the matrix elements are all eventually
scaled so they have a maximum value of less than one. In addition,
when the decoder is finally put to use, the gain of each output to
the loudspeaker is adjusted. To adjust the gain of each output, a
signal which has been encoded from the four major directions (left,
center, right, and surround) with equal sound power is played, and
the gain of each output is adjusted until the sound power is equal
in the listening position. In practice, this means that the actual
level of the matrix elements is scaled so the four outputs of the
decoder are equal under conditions of full steering. This
calibration has been explicitly included in the equations for the
rear elements above.
The 3 dB difference in the elements in the forward steered or
unsteered condition is not trivial. During unsteered conditions,
the elements from the '89 patent have the value 0.71, and the sum
of the squares of the elements has the value of one. This is not
true of the calibrated Dolby.RTM. rear elements. LRL has the
unsteered value of one, and the sum of the squares is 2, which is 3
dB higher than the outputs in the '89 patent. Note that the
calibration procedure results in a matrix that does not correspond
to the "Dolby.RTM. Surround.RTM." passive matrix when the matrix is
unsteered. The Dolby.RTM. Surround.RTM. passive matrix specifies
that the rear output should have the value of 0.71*(A.sub.in
-B.sub.in), and the Dolby.RTM. Pro-Logic.RTM. matrix does not meet
this specification. As a result, the rear output will be 3 dB
stronger than the others when the A and B inputs are decorrelated.
If there are two speakers sharing the rear output, each will be
adjusted to be 3 dB softer than a single rear speaker, which will
make all five speakers have approximately equal sound power when
the decoder inputs are uncorrelated. When the matrix elements from
the '89 patent are used, the same calibration procedure results in
3 dB less sound power from the rear when the decoder inputs are
uncorrelated.
The issue of how loud the rear channels should be when the inputs
are decorrelated is a matter of taste. When a surround encoded
recording is being played, it may be desirable to reproduce the
balance heard by the producer when the recording was mixed.
Achieving this balance is a design goal for the decoder and encoder
as a combination. However, with standard stereo material, the goal
is to reproduce the power balance in the original recording, while
generating a tasteful and unobtrusive surround. The problem with
the Dolby.RTM. matrix elements is that the power balance in a
conventional two channel recording is not preserved through the
matrix, in that the surround channels are too strong, and the
center channel is too weak.
To see the importance of this issue, consider what happens when the
input to the decoder consists of three components, an uncorrelated
left and right component, and a separate and uncorrelated center
component.
When A.sub.in and B.sub.in are played through a conventional stereo
system, the sound power in the room will be proportional to
L.sub.in.sup.2 +R.sub.in.sup.2 +C.sub.in.sup.2. If all three
components have roughly equal amplitudes, the power ratio of the
center component to the left plus right component will be 1:2.
It may be desirable for the decoder to reproduce sound power in the
room with approximately the same power ratio as stereo, regardless
of the power ratio of C.sub.in to L.sub.in and R.sub.in. This can
be expressed mathematically. Essentially, the equal power ratio
requirement will specify the functional form of the center matrix
elements along the cs axis, if all the other matrix elements are
taken as given. If it is assumed that the Dolby.RTM. matrix
elements, calibrated such that the rear sound power is 3 dB less
than the other three outputs when the matrix is fully steered (i.e.
3 dB less than the standard calibration), then the center matrix
elements should have the shape shown in FIG. 16. If the same thing
is done for the standard calibration, the results in FIG. 17
emerge.
In FIG. 16, the solid curve shows the values of the center matrix
elements as a function of cs assuming the power ratios in the
decoder outputs are identical to the power ratios in stereo, and
using the rear Dolby.RTM. matrix elements calibrated 3 dB lower in
level than is typically used. The dotted curve shows the actual
value of the center matrix elements in Pro-Logic. While the actual
value gives reasonable results for an unsteered signal and a fully
steered signal, the actual value is about 1.5 dB too low in the
middle.
In FIG. 17, the solid curve shows the value of the center matrix
elements assuming equal power ratios to stereo given the matrix
elements and the calibration actually used in Dolby.RTM. Pro-Logic.
The dotted curve shows the actual values of the center matrix
elements in Pro-Logic.RTM.. The actual values are more than 3 dB
too low for all values of cs.
These two figures show something of which mix engineers are often
aware--that a mix prepared for playback on a Dolby.RTM.
Pro-Logic.RTM. system can require more center loudness than a mix
prepared for playback in stereo. Conversely, a mix prepared for
stereo playback will lose vocal clarity when played over a
Dolby.RTM. Pro-Logic.RTM. decoder. Ironically, this is not true of
a passive Dolby.RTM. Surround.RTM. decoder.
14. Creating two Independent Rear Outputs
The major problem with both the elements of the '89 patent and the
elements of the Dolby.RTM. Pro-Logic.RTM. decoder is that there is
only a single rear output. The '92 patent disclosed a method for
creating two independent side outputs, and the math in the '92
patent was incorporated in the elements of the front left quadrant
of reference [1] and the November '96 application. The goal for the
elements in this quadrant was to eliminate the output of a signal
steered from left to center, while maintaining some output from the
left rear channel for unsteered material present at the same time.
To achieve this goal, it was assumed that the LRL matrix element
would have the following form for the left front quadrant:
These matrix elements are very similar to the elements in the '89
patent, but further include a G(lr) term in LRR, and a GS term in
LRL. G(lr) was included to add signals from the B input channel of
the decoder to the left rear output to provide some unsteered
signal power as the steered signal was being removed. GS(lr) was
determined according to the criterion that there should be no
signal output with a fully steered signal that is moving from left
to center. The formula for GS(lr) was determined to be equal to
G.sup.2 (lr). However, a more complicated representation of the
formula is given in the '92 patent. The two representations can be
shown to be identical.
In reference [1] these elements are corrected by a boost of
(sin(cs)+cos(cs)) so that they more closely approximate constant
loudness for unsteered material. While completely successful in the
right front quadrant, this correction is not very successful in the
left front quadrant. As shown in FIG. 18, the matrix elements are
identical to the LRL and LRR elements in the '89 patent for the
right front quadrant. In FIG. 18, there is a 3 dB dip along the
line from the middle to the left vertex in the front left quadrant,
and nearly a 3 dB boost in the level along the boundary between
left and center. The "mountain range" in the rear quadrant will be
discussed later. For the plot shown in FIG. 18, the "tv matrix"
correction in V1.11 has been removed to allow better comparison to
the present invention, which is shown in FIG. 20.
Several problems with the sound power are shown in FIG. 18. For
example, there is a dip in the sum of the squares along the cs=0
axis. This dip exists because the functional shape of G(lr) in LRR
is not optimal. In fact, the choice of G(lr) was arbitrary. This
function already existed in an earlier design of the decoder, and
was easily implemented in analog circuitry.
It may b e desirable to have a function GR(lr) in this equation,
choose GS(lr) and GR(lr) in such a way as to keep the sum of the
squares of LRL and LRR constant along the cs=0 axis, and keep the
output zero along the boundary between left and center. It may also
be desirable for the matrix elements to be identical to the matrix
elements in the right front quadrant along the lr=0 axis. It is
assumed that:
So that the sum of the squares are one along the cs=0 axis:
and so that the output is zero for a steered signal, or as t varies
from zero to 45 degrees:
When solving for GR(lr) and GS(lr), equations (18) and (19) result
in a messy quadratic equation, which is solved numerically and
shown in FIG. 19. As intended, use of the values obtained for GS
and GR, as shown in FIG. 19, results in a large improvement in the
power sum along the cs=0 axis. However, the peak in the sum of the
squares along the boundary between left and center (shown in FIG.
18) remains.
In a practical design it is probably not very important to
compensate for this error. However, this compensation may be
accomplished heuristically by dividing both matrix elements by a
factor that depends on a new combined variable ("xymin") that is
based on lr and cs. Alternatively, both matrix elements may be
multiplied by the inverse of xymin. For example, in Matlab
notation: % find the minimum of x or y xymin=x; if (xymin>y)
xymin=y; end if (xymin>23) xymin=23; end % note that xymin
varies from zero to 22.5 degrees.
The correction to the matrix elements along the boundary may be
found using xymin. In the front left quadrant:
In the front right quadrant:
In reference [2], these elements are also multiplied by the "tv
matrix" correction. FIG. 20 shows the matrix elements without the
"tv matrix" correction. The "tv matrix" correction is handled by
frequency dependent circuitry that follows the matrix, which will
be described later. As shown in FIG. 20, the sum of the squares is
close to one and continuous, except for the deliberate rise in
level in the rear.
15. The Rear Matrix Elements During Rear Steering
The rear matrix elements given in the '92 patent were not
appropriate for a five-channel decoder, and, therefore, may be
modified heuristically. Reference [1] and the November '96
application presented a mathematical method for deriving these
elements along the boundary of the left rear quadrant. The method
worked along the boundary, but resulted in discontinuities along
the lr=0 axis, and the cs=0 axis. These discontinuities were mostly
repaired by additional corrections to the matrix elements, which
preserved the behavior of the matrix elements along the steering
boundaries.
These discontinuties may also be corrected using interpolation. A
first interpolation fixes discontinuities along the cs=0 boundary
for LRL. This interpolation causes the value of LRL to match the
value of GS(lr) when cs is zero, and allows the value of LRL to
rise smoothly to the value given by the previous math as cs
increases negatively toward the rear. A second interpolation causes
the value of LRR to match the value of GR(lr) along the cs=0
axis.
16. Left Side/Rear Outputs During Rear Steering From Right to Right
Rear
Consider the LRL and LRR matrix elements when the steering is
neutral or anywhere between full right and right rear (lr can vary
from 0 to -45 degrees, and cs can vary from 0 to -22.5 degrees).
Under these conditions, the steered component of the input should
be removed from the left outputs, which means there should be no
output from the rear left channel when the steering is toward the
right or right rear.
The matrix elements given in the '92 patent achieve this goal and
are essentially the same as the rear matrix elements in a 4 channel
decoder with the addition of a sin(cs)+cos(cs) correction for the
unsteered loudness. Therefore, the matrix elements are simple sines
and cosines and are defined according to the following
equations:
where sric(x) is equal to sin(x) over a value with a range of 0 to
22.5 degrees, and sri(x) is equal to cos(x). These functions will
also be used to define the Left Rear matrix elements during Left
steering.
17. Left Side and Rear Outputs During Rear Steering From Right Rear
to Rear
Consider the same matrix elements as cs becomes greater than -22.5
degrees (cs varies from -22.5 to -45). As stated in reference [1],
the July '96 application and the November '96 application, LRL
should rise to one or more over this range, and LRR should decrease
to zero. Simple functions fulfill these requirements:
where rboost(cs) is defined in reference [1] and the November '96
application. rboost(cs) is closely equivalent to the function
0.41*G(cs) in the earlier matrix elements, except that rboost(cs)
is zero for 0>cs>-22.5, and varies from zero to 0.41 as cs
varies from -22.5 degrees to -45 degrees. The exact functional
shape of rboost(cs) is determined by the desire to keep the
loudness of the rear output constant as sound is panned from left
rear to full rear. The Left Rear matrix elements during right
steering are now complete.
18. The Left Rear Matrix Elements During steering From Left to Left
Rear
The behavior of the LRL and LRR matrix elements is complex. The LRL
element must quickly rise from zero to near maximum as lr decreases
from 45 to 22.5 or to zero. The matrix elements given in reference
[1] satisfy this requirement, but as shown previously, there are
problems with continuity at the cs=0 boundary.
One solution to the continuity problems uses functions of one
variable and several conditionals. In reference [1], the problem at
the cs=0 boundary arises because the LRL matrix element is given by
GS(lr) on the forward side of the boundary (cs>0). On the rear
side of the boundary (cs<0), the function given by reference [1]
has the same end points, but is different when lr is not zero or 45
degrees.
The mathematical method in reference [1] provides the following
equations for the Left Rear matrix elements over the range
22.5<lr<45 (in reference [1],t=45-lr): ##EQU1##
where sra(lr) and srac(lr) are two new functions defined over this
range.
If cs.gtoreq.22.5, lr can still vary from 0 to 45. Reference [1]
defines LRL and LRR (when the range of lr is 0<lr<22.5; see
FIG. 6 in reference [1]), respectively, as:
which defines the two functions sra(x) and srac(x) for
0<lr<45.
19. March 1997 Version
There are two discontinuities in the March 1997 version. Along the
cs=0 boundary, the LRR for the rear must match the LRR for the
forward direction, which shows LRR=-G(lr) along the cs=0 boundary.
A somewhat computationally intensive interpolation, which is based
on cs over the range of values of 0 to 15 degrees, is used to
correct LRR. When cs is zero G(lr) is employed to find LRR and as
cs increases to 15 degrees, LRR is interpolated to the value of
srac(lr).
A discontinuity along the lr=0 axis is also possible. This
discontinuity was corrected somewhat by adding a term to LRR, which
is found by using a new variable ("cs_bounded"). The correction
term becomes simply sric(cs_bounded), which will insure continuity
across the lr=0 axis. cs_bounded may be defined according to the
following Matlab notation: cs_bounded=lr-cs; if (cs_bounded<1) %
this limits the maximum value cs_bounded=0; end if
(45-.vertline.lr.vertline.<cs_bounded) % use the smaller of the
two values cs_bounded=45-lr; end for cs=0 to 15
LRR=(-(srac(lr)+(srac(lr)-G(lr))*(15-cs)/15)+sric(cs_bounded)); for
cs=15 to 22.5 LRR=(-srac(lr)+sric(cs_bounded));
20. LRL as implemented in the present invention
In the present invention, LRL is computed using an interpolation
similar to that used for LRR. In Matlab notation: for cs=0 to
15
LRL=((sra(lr)+(sra(lr)-GS(lr))*(15-cs)/15)+sri(-cs)); for cs=15 to
22.5
21. Rear Outputs During Steering From Left Rear to Full Rear
As the steering goes from left rear to full rear the elements
follow those given in reference [1], however, corrections for rear
loudness are added. In Matlab notation: For cs>22.5,
lr<22.5
This completes the LRL and LRR matrix elements during left
steering. The values for right steering can be found by swapping
left and right in the definitions.
22. Center Matrix Elements
The '89 patent and Dolby.RTM. Pro-Logic.RTM. both have center
matrix elements defined by equations (24a), (24b), (24c) and (24d).
For front steering:
For rear steering:
CL=1-G(lr) (24c)
Because the matrix elements have symmetry about the left/right
axis, the values of CL and CR for right steering can be found by
swapping CL and CR. FIG. 21 shows a graphical representation of CL,
in which the middle of the graph and the right and rear vertices
have the value 1, and the center vertex has the value 1.41. In
practice, this element is scaled so that its maximum value is
one.
In the November '96 application and reference [1], these elements
are defined by sines and cosines according to equations (25a) and
(25b). For front steering:
However, the March 1997 version used the elements defined in the
'89 patent, but with a different scaling, and a boost function
different than G(cs). It was important to reduce the unsteered
level of the center output, therefore, a value 4.5 dB less than the
value used in Dolby.RTM. Pro-Logic was chosen and the boost
function (0.41*G(cs)) was changed to increase the value of the
matrix elements back to the value used in Dolby.RTM. Pro-Logic.RTM.
as cs increases toward center. The boost function in the March 1997
version was chosen heuristically through listening tests.
In the March 1997 version, the boost function of cs starts at zero
as before, and increases with cs such that CL and CR increase by
4.5 dB as cs goes from zero to 22.5 degrees. The increase in CL and
CR is a constant number (in dB) for each dB of increase in cs. The
boost function then changes slope such that the matrix elements
increase another 3 dB in the next 20 degrees and then remain
constant. Thus, the new matrix elements are equal to the neutral
values of the old matrix elements when the steering is "half front"
(8 dB or 23 degrees). As the steering continues to move forward,
the new and the old matrix elements become equal. The output of the
center channel is thus 4.5 dB lower than the old output when
steering is neutral, but increases to the old value when the
steering is fully to the center. FIG. 22 shows a three-dimensional
plot of the CL matrix element. In this plot, the middle value and
the right and rear vertices have been reduced by 4.5 dB.
Addionally, as cs increases, the center rises to the value of 1.41
in two slopes.
However, the center elements used in the March 1997 version are not
optimal. Considerable experience with the decoder in practice has
shown that the center portion of popular music recordings and the
dialog in some films tends to get lost when switching between
stereo (two channel) reproduction, and reproduction using the
matrix. In addition, a listener who is not equidistant from the
front speakers can notice the apparent position of a center voice
moving as the level of the center channel changes. This problem was
extensively analyzed as the new center matrix elements presented
here were developed. There is also a problem when a signal pans
from left to center or from right to center along the boundary. The
matrix elements given in the November '96 application result in a
center speaker output that is too low when the pan is half way
between.
23. Center Channel in the New Design
While it is possible to remove a strongly steered signal from the
center channel output using matrix techniques, any time the
steering is frontal but not biased either left or right, the center
channel must reproduce the sum of the A and B inputs with some gain
factor. In other words, it is not possible to remove uncorrelated
left and right material from the center channel. The only option is
to regulate the loudness of the center speaker.
How loud the center speaker should be depends on the behavior of
the left and right main outputs. The matrix values presented above
for LFL and LFR are designed to remove the center component of the
input signals as the steering moves forward. If the input signal
has been encoded to come from the forward direction using a cross
mixer, such as a stereo width control, the matrix elements given
above (the elements of the '89 patent, reference [1], the March
1997 version, and those presented earlier in this paper) completely
restore the original separation.
However, the input to the decoder may consist of uncorrelated left
and right channels to which an unrelated center channel has been
added. For example, the input channels may be defined according to
the following equations:
When this is the case, as the level of C.sub.in increases relative
to L.sub.in and R.sub.in the C component of the L and R front
outputs of the decoder is not completely eliminated unless C in is
large compared to L.sub.in and R.sub.in. In general, a bit of C in
remains in the L and R front outputs. However, what will a listener
hear?
There are two ways of calculating what a listener hears depending
on whether the listener is exactly equidistant from the Left,
Right, and Center speakers. If a listener is exactly equidistant
from the Left, Right, and Center speakers, they will hear the sum
of the sound pressures from each speaker. This is equivalent to
summing the three front outputs. When the listener is in this
position, any reduction of the center component of the left and
right speakers will result in a net loss of sound pressure from the
center component, regardless of the amplitude of the center
speaker. This net loss of sound pressure from the center component
is a result of deriving the signal in the center speaker from the
sum of the A and B inputs. Therefore, as the amplitude of the
signal in the center speaker is raised, the amplitude of the
L.sub.in and R.sub.in signals must rise along with the amplitude of
the C.sub.in signal.
However, if the listener is not equidistant from each speaker, the
listener is much more likely to hear the sum of the sound power
from each speaker, which is equivalent to the sum of the squares of
the three front outputs. In fact, extensive listening has shown
that the sum of the sound power from each speaker is actually what
is important. Therefore, the sum of the squares of all the outputs
of the decoder, including the rear outputs, must be considered.
To design the matrix so that the ratio of the amplitudes of
L.sub.in, R.sub.in, and C.sub.in are preserved when switching
between stereo reproduction and matrix reproduction, the sound
power of the C.sub.in component from the center output must rise in
exact proportion to the reduction in the sound power of the
C.sub.in component from the left and right outputs, and the
reduction in the sound power of the C.sub.in component in the rear
outputs. An additional complication comes from the up to 3 dB level
boost applied to the left and right front outputs (described
previously). Because of the level boost, the center will need to be
somewhat louder to keep the ratios constant. This requirement may
be expressed as a set of equations for the sound power. Using these
equations, a gain function, which can be used to increase the
loudness of the center speaker, can be determined.
The solid curve of FIG. 23 shows the center gain needed to preserve
the energy of the center component of the input signal in the front
three channels as steering increases toward the front. The dotted
curve of FIG. 24 shows the gain in a standard decoder. As shown by
the solid curve, the level of the center channel requires a steep
increase- on the order of many dB of amplitude per dB of steering
value.
As previously mentioned, there are two solutions to this problem.
One solution is the "film" solution, which is not entirely
mathematical. The function shown in FIG. 23 rose too steeply, in
that the change in level of the center channel was too obvious.
Therefore, the power requirement was relaxed slightly so that the
power in the center was about 1 dB less than the ideal. The relaxed
power requirement may be used to recalculate the center values,
which are indicated by the solid line of FIG. 24. In practice a
linear rise can be substituted for the early part of the curve, as
indicated by the dashed line in FIG. 24. These center values have
yielded excellent results for films. Because the curve indicated by
the solid line in FIG. 24 rises to steeply, the linear slope
indicated by the dashed line works better.
In contrast, music requires a different solution. The center
attenuation shown in FIGS. 23 and 24 was derived using the matrix
elements previously given for LFL and LFR. However, what if
different elements were used? Specifically, would the center
component need to be aggressively removed from the left and right
front outputs?
Listening tests show that the previous left and right front matrix
elements are needlessly aggressive about removing the center
component during music playback. Acoustically there is no need.
Energy removed from the left and right front must be given to the
center loudspeaker. If, however, this energy is not removed, it
will come from the left and right front speakers, and, therefore,
the center speaker need not be as strong and the sound power in the
room remains the same. The trick is to put just enough energy into
the center speaker to create a convincing front image for an
off-axis listener, while minimizing the reduction of stereo width
for a listener who is equidistant from the front left and right
speakers.
As done in the November '96 application,the optimal center loudness
can be found by trial and error. The matrix elements needed in the
front left and right to preserve the power of the C.sub.in
component in the room may then be determined. As before, it is
assumed that the center channel is reduced in level by 4.5 dB below
the level in the decoder disclosed in the '89 patent, which is a
total attenuation of -7.5 dB total attenuation, which is about
0.42. The matrix elements for the center can be multiplied by this
factor, and a new center boost function (GC) can be defined.
For front steering:
For rear steering:
Several functions were tried for GC(cs). The function given below
may not be ideal, but seems good enough. The function is specified
in terms of the angle cs in degrees, and was obtained by trial and
error.
In MATLAB notation: center_max=0.65; center_rate=0.75;
center_max2=1; center_rate2=0.3; center_rate3=0.1; if (cs<12)
gc(cs-1)=0.42*10, (db*center_rate/(20)); tmp=gc(cs+1); elseif
(cs<30) gc(cs+1)=tmp*10 ((cs-11)*center_rate3/(20)); if
(gc(cs+1)>center_max) gc(cs+1)=center_max; end else
gc(cs+1)=center_max*10 ((cs-29)*center_rate2/(20)); if
(gc(cs+1)>center_max2) gc(cs+1)=center_max2; end end
The function (0.42+GC(cs)) is plotted in FIG. 25. Note the quick
rise from the value 0.42 (4.5 dB lower than Dolby.RTM.
Surround.RTM.), followed by a gentle rise, and finally by a steep
rise to the value 1.
The function needed for LFR may be determined if functions for LFL,
LRL, and LRR are assumed. This involves determining the rate at
which the C.sub.in component in the left and right outputs should
decrease, and then designing matrix elements that provide this rate
of decrease. These matrix elements should also provide some boost
of the L.sub.in and R.sub.in components, and should have the
current shape at the left to center boundary, as well as the right
to center boundary.
It is assumed that:
Power from the front left and right can then be computed as
follows:
Power from the center is:
Power from the rear depends on the matrix elements used. It was
assumed that the rear channels are attenuated by 3 dB during
forward steering, and that LRL is cos(cs) and LRR is sin(cs). From
a single speaker:
If it is assumed that L.sub.in.sup.2.apprxeq.R.sub.in.sup.2, then,
for two speakers:
The total power from all three speakers is PLR+PC+PREAR:
The ratio of C.sub.in power to L.sub.in and R.sub.in power
(assuming L.sub.in.sup.2 =R.sub.in.sup.2) is:
For normal stereo, GC=0, GP=1, and GF=0. Therefore, the center to
LR power ratio is:
RATIO=(C.sub.in.sup.2 /L.sub.in.sup.2) *0.5 (32)
If this ratio is to be constant regardless of the value of
C.sub.in.sup.2 /L.sub.in.sup.2 for the active matrix, then:
The equation above can be solved numerically. Assuming the GC
above, and GP=LFL as before, the result is shown in FIG. 26. In
FIG. 26 the solid curve is the GF needed for constant energy ratios
with the new "music" center attenuation GC. The dashed curve is the
LFR element of the March '97 version (sin(cs)*corrl). The dotted
curve is sin(cs), which is the LFR element without the correction
term corrl. Note that GF is close to zero until cs reaches 30
degrees, and then GF increases sharply. In practice it is best to
limit the value of cs to about 33 degrees. In practice, the LFR
element derived from these curves has a negative sign.
GF gives the shape of the LFR matrix element along the lr=0 axis,
as cs increases from zero to center. A method is needed of blending
this behavior to that of the previous LFR element, which must be
preserved along the boundary between left and center, as well as
from right to center. A method of doing this when cs<22.5
degrees is to define a difference function between GF and sin(cs).
This function may then be limited in various ways. In Matlab
notation: gf_diff=sin(cs)-gf(cs): for cs=0:45; if
(gf_diff(cs)>sin(cs)) gf_diff(cs)=sin(cs); end if
(gf_diff(cs)<0) gf_diff(cs)=0; end end % find the bounded c/s if
(y<24) bcs=y-(x-1); if (bcs<1) % this limits the maximum
value bcs=1; end else bcs=47-y-(x-1); if (bcs<1) %>46) bcs=1;
%46; end end
The LFR element can now be written in Matlab notation: % this neat
trick does an interpolation to the boundary % the cost, of course,
is a divide!!! if (y<23) % this is the easy way for half the
region lfr3d(47-x,47-y)=-sin_tbl(y)+gf_diff(bcs); else
tmp-((47-1-x)/(47-1))*gf_diff(y); lfr3d(47-x,47-y)=-sin_tbl(y)+tmp;
end
Note that the sign of gf_diff is positive in the equation above.
Thus gf_diff cancels the value of sin(cs), reducing the value of
the element to zero along the first part of the lr=0 axis, as shown
in FIG. 27.
In FIG. 27, the value is zero in the middle of the plane (where
there is no steering) and remains zero as cs increases to .about.30
degrees along the lr=0 axis. The value then falls off to match the
previous value along the boundary from left to center and from
right to center.
24. Panning Error in the Center Output
The new center function may be written as follows:
As defined in equations (34a) and 34(b), the new center function
works well along the lr=0 axis, but causes a panning error along
the boundary between left and center, and between right and center.
However, the values in reference [1] give a smooth function of
cos(2*cs) along the left boundary and create smooth panning between
left and center. It is desirable for the new center function to
have similar behavior along this boundary.
A correction to the matrix element that will do the job includes
adding an additional function "xymin", which may be expressed in
Matlab notation as: center_fix_tbl=0.8*(corrl-1);
Then:
A three-dimensional representation of the CL matrix element is
shown in FIG. 28. While not perfect, this correction works well in
practice. In FIG. 28, note the correction for panning along the
boundary between left and center, which is fairly smooth.
FIG. 29 shows a graph of the left front (dotted curve) and center
(solid curve) outputs, where the center steering is to the left of
the plot, and full left is to the right. In the "music" strategy,
the value of cs is limited to about 33 degrees (about 13 on the
axis as labeled), where the center is about 6 dB stronger than the
left.
25. Technical Details of the Encoder
There are two major goals for the Logic 7.RTM. encoder. First, the
Logic 7.RTM. encoder should be able to encode a 5.1 channel tape in
a way that allows the encoded version to be decoded by a Logic
7.RTM. decoder with minimal subjective change. Second, the encoded
output should be stereo compatible, which means that it should
sound as close as possible to a manual two channel mix of the same
material. Stereo compatibility should include the output of the
encoder giving identical perceived loudness for each sound source
in an original 5 channel mix when played on a standard stereo
system. The apparent position of the sound source in stereo should
also be as close as possible to the apparent position of the sound
source in the 5 channel original.
The goal of stereo compatibility, as described above, cannot be met
by a passive encoder. A five channel recording where all channels
have equal foreground importance must be encoded as described
above. This encoding requires that surround channels be mixed into
the output of the encoder in such a way as to preserve the energy.
That is, the total energy of the output of the encoder should be
the same, regardless of which input is being driven. This constant
energy setting will be necessary for most film sources and for five
channel music sources where instruments have been assigned equally
to all 5 loudspeakers, although such music sources are not common
at the present time, they will become common in the future.
Music recordings in which the foreground instruments are placed in
the front three channels, and reverberation is placed primarily in
the rear channels, require a different encoding. Music recordings
of this type were successfully encoded in a stereo compatible form
when the surround channels were mixed with 3 dB less power than the
other channels. This -3 dB level has been adopted as a standard for
surround encoding in Europe. However, the European standard
specifies that other surround levels can be used for special
purposes. The new encoder contains active circuits, which detect
strong signals in the surround channels. When the active circuits
detect that such signals are occasionally present, the encoder uses
full surround level. If the active circuits detect that the
surround inputs are consistently -6 dB or less compared to the
front channels, the surround gain is gradually lowered 3 dB, which
corresponds to that of the European standard.
These active circuits were also present in the encoder in the
November '96 application. However, tests involving the encoder of
the November '96 application, performed at the Institute for
Broadcast Technique (IRT) in Munich, revealed that the direction of
some sound sources was encoded incorrectly. Therefore, a new
architecture was developed to solve this problem. The new encoder
is clearly superior in its performance on a wide variety of
difficult material. The original encoder was developed first as a
passive encoder. The new encoder will also work in a passive mode,
but is primarily intended to work as an active encoder. The active
circuitry corrects several small errors inherent in the design.
However, even without the active correction, the performance is
better than the previous encoder.
Through extensive listening, several other small problems with the
first encoder were discovered. Many of these problems have been
addressed in the new encoder. For example, when stereo signals are
applied to both the front and the rear terminals of the encoder at
the same time, the resulting encoder output is biased too far to
the front. The new encoder compensates for this by increasing the
rear bias slightly. Likewise, when a film is encoded with
substantial surround content, dialog can sometimes get lost. This
problem was greatly improved by the changes to the power balance
described above. However, the encoder is also intended for use with
a standard (Dolby.RTM.) decoder and compensates for this by raising
the center channel input to the encoder slightly when used in this
manner.
26. Explanation of the Design
The new encoder handles the left, center, and right signals in a
manner identical to that of the previous design and the Dolby.RTM.
encoder, providing that the center attenuation function
.function.cn is equal to 0.71, or -3 dB.
The surround channels look more complicated than they are. The
functions .function.c( ) and .function.s( ) direct the surround
channels either to a path with a 90 degree phase shift relative to
the front channels, or to a path with no phase shift. In the basic
operation of the encoder, .function.c is one, and .function.s is
zero, which means that only the path which uses the 90 degree phase
shift is active.
crx controls the amount of negative cross feed for each surround
channel and is typically 0.38. As in the previous encoder, the A
and B outputs have an amplitude ratio of -0.38/0.91 when there is
only an input to one of the surround channels. The amplitude ratio
results in a steering angle of 22.5 degrees to the rear. As usual,
the total power in the two output channels is unity (the sum of the
squares of 0.91 and 0.38 is one).
While the output of this encoder is relatively simple when only one
channel is driven, it becomes problematic when both surround inputs
are driven at the same time. If the LS a nd the RS input are driven
with the same signal (a common occurrence in film), all the signals
at the summing nodes are in phase, so the total level in the output
channels is 0.38+0.91, which is 1.29. This output level is too
strong by the factor of 1.29, which is 2.2 dB. Therefore, active
circuitry is included in the encoder that reduces the value of the
function .function.c by up to 2.2 dB when the two surround channels
are similar in level and phase.
Another error occurs when the two surround channels are similar in
level and out of phase. In this case, the two attenuation factors
subtract, so the A and B outputs have equal amplitude and phase,
and a level of 0.91-0.38, which is 0.53. This signal will be
decoded as a center direction signal, which is a severe error. The
previous encoder design produced an unsteered signal under these
conditions, which is reasonable. However, it is not reasonable that
signals applied to the rear input terminals result in a center
oriented signal. Thus, active circuitry is supplied, which
increases the value of .function.s when the two rear channels are
similar in level and antiphase. Mixing both the real path and the
phase shifted path for the rear channels results in a 90 degree
phase difference between the output channels A and B. This results
in an unsteered signal, which is desired.
As previously mentioned, a surround encoder using the European
standard attenuates the two surround channels by 3 dB and adds them
into the front channels. Thus, the left rear channel is attenuated
and added to the left front channel. A surround encoder using the
European standard has many disadvantages when encoding multichannel
film sound or recordings that have specific instruments in the
surround channels. One such advantage is that both the loudness and
the direction of these instruments will be incorrectly encoded.
However, ae surround encoder using the European standard works
rather well with classical music, for which the two surround
channels are primarily reverberation. The 3 dB attenuation of the
European standard was carefully chosen through listening tests to
produce encoding that is stereo-compatible. Therefore, the new
encoder should include this 3 dB attenuation when classical music
is being encoded. The presence of classical music can be detected
through the relative levels of the front channels and the surround
channels in the encoder.
A major function of the function .function.c in the surround
channels is to reduce the level of the surround channels in the
output mix by 3 dB when the surround channels are much softer than
the front channels. Circuitry is provided to compare the front and
rear levels, and reduce the value of .function.c to a maximum of 3
dB when the rear levels are 3 dB less than the front levels.
Maximum attenuation is reached when the rear channels are 8 dB less
strong than the front channels. This active circuit appears to work
well and makes the new encoder compatible with a surround encoder
using the European standard for classical music. The action of the
active circuits causes instruments, which are intended to be strong
in the rear channels, to be encoded with full level.
The real coefficient mixing path .function.s has another function
for the surround channels. When a sound is moving from the left
front input to the left rear input, active circuitry detects when
these two inputs are similar in level and in phase. Under these
conditions, .function.c is reduced to zero and .function.s is
increased to one. This change to real coefficients in the encoding
results in a more precise decoding of this type of pan. In
practice, this function is probably not essential, but seems to be
an elegant refinement.
There is an additional active circuit- a level detecting circuit.
Level detecting circuits look at the phase relationship between the
center channel and the front left and right. Some popular music
recordings that use five channels mix the vocals into all three
front channels. When there is a strong signal in all three inputs,
the encoder output will have excessive vocal power, because the
three front channels will add together in phase. When this occurs,
active circuits increase the attenuation in the center channel by 3
dB to restore the power balance in the encoder output.
In summary, active circuits are provided to: 1. Reduce the level of
the surround channels by 2.2 dB when the two channels are in phase;
2. Sufficiently, increase the real coefficient mixing path for the
rear channels to create an unsteered condition when the two rear
channels are out of phase; 3. Decrease the level of the surround
channels by up to 3 dB when the surround level is much lower than
the front levels; 4. Increase the level and negative phase of the
rear channels when the level of the rear channels is similar to the
level of the front channels; 5. Cause the surround channel mix to
use real coefficients when a sound source is panning from a front
input to the corresponding rear input; 6. Increase the level of the
center channel in the encoder when the center level and the level
of the front and surround inputs are approximately equal; and 7.
Decrease the level of the center channel in the encoder when a
there is a common signal in all three front inputs.
27. Frequency dependent circuits in the decoder
FIG. 2 is a block diagram that includes frequency dependent
circuits that follow the matrix in a five channel version of the
decoder. The frequency dependent circuits include three sections: a
variable low pass filter, a variable shelf filter, and a HRTF (Head
Related Transfer Function) filter. The HRTF filter changes its
characteristics depending on the value of the rear steering voltage
c/s. The first two filters change their characteristics in response
to a signal that is intended to represent the average direction of
the input signals to the decoder during pauses between strongly
steered signals. This signal is called the background control
signal.
28. The Background Control Signal
One of the major goals of the current decoder is to optimally
create a five channel surround signal from an ordinary two channel
stereo signal. It is also highly desirable for the decoder to
recreate a five channel surround recording that was encoded into
two channels by the encoder described in this application. These
two goals differ in the way in which the surround channels are
perceived. With an ordinary stereo input, the majority of the sound
needs to be in front of the listener. The surround speakers should
contribute a pleasant sense of envelopment and ambience, but should
not draw attention to themselves. With an encoded surround
recording, the surround speakers need to be stronger and more
aggressive.
To play both types of input optimally without any adjustment by the
user, it is necessary to discriminate between a two channel
recording and an encoded five channel recording. The background
control signal is designed to make this discrimination. The
background control signal ("BCS") is similar to and derived from
the rear steering signal cs. BCS represents the negative peak value
of cs. That is, when cs is more negative than BCS, BCS is made to
equal cs. When cs is more positive than BCS, BCS slowly decays.
However, the decay of BCS involves a further calculation.
Music of many types consists of a series of strong foreground
notes, or in the case of a song, sung words. There is a background
between the foreground notes that may consist of other instruments
playing other notes or reverberation. The circuit that derives the
BCS signal keeps track of the peak level of the foreground notes.
When the current level is -7 dB less than the peak level of the
foreground, the level of cs is measured. The value of cs during the
gaps between foreground peaks is used to control the decay of BCS.
If the material in the gaps is reverberation, cs may tend to have a
net rearward bias in a recording that was made by encoding a five
channel original. This is because the reverberation on the rear
channels of the original will be encoded with a rearward bias. The
reverberation in an ordinary two channel recording will have no net
rearward bias. cs for this reverberation will be zero or slightly
forward.
BCS derived in this way tends to reflect the type of recording. Any
time there is significant rear steered material, BCS will always be
strongly negative. However, BCS can be negative even in the absence
of strong steering to the rear if the reverberation in the
recording has a net rearward bias. The filters that optimize the
decoder for stereo versus surround inputs may be adjusted using
BCS.
29. Frequency dependent circuits: five channel version
The first of the filters in FIG. 2 is a simple 6 dB per octave low
pass filter with an adjustable cutoff frequency. This filter is set
to a value that is user adjustable when BCS is positive or zero,
but is typically about 4 kHz. The cutoff frequency of the filter is
raised as BCS becomes negative until BCS is more rearward than 22
degrees. At this point, the filter is not active. This low
frequency filter makes the rear outputs less obtrusive when
ordinary stereo material is played. In earlier decoders the filter
was controlled by cs, and not by BCS.
The second filter is a variable shelf filter that implements the
"sound stage" control in the current decoder. In the November '96
application, the "soundstage" control was implemented through the
matrix elements using the "tv matrix" correction. The earlier
decoders reduced the overall level of the rear channels when the
steering was neutral or forward. In the new decoder, the matrix
elements do not include the "tv matrix" correction. The second
filter of FIG. 2 includes a low frequency section (the pole) that
is fixed at 500 Hz and a high frequency section (the zero) that
varies depending on user adjustment and BCS.
The high frequency section of the shelf filter is set equal to the
low frequency section when the soundstage control is set to "rear"
in the new decoders. In other words, the shelf has no attenuation,
and the filter has flat response. However, the setting of the high
frequency zero varies when the soundstage control is set to
"neutral" in the new decoders. The zero moves to 710 Hz when BCS is
positive or zero, resulting in a 3 dB attenuation of higher
frequencies. The result is the same as that of the earlier decoders
for the high frequencies. There is a 3 dB attenuation when the
steering is neutral or forward. However, the low frequencies are
not attenuated and come from the sides of the room with full level.
This results in greater low frequency richness and envelopment,
without the distracting high frequencies in the rear. The high
frequency zero moves toward the pole as BCS becomes negative so
that the shelf filter has an attenuation when BCS is about 22
degrees to the rear. While the action is similar when the
soundstage control is set to "front", but the zero moves to 1 kHz
when BCS is zero or positive. This gives the high frequencies an
attenuation of 6 dB. Once again, the attenuation is removed as BCS
goes negative.
The third filter is controlled by c/s and not by BCS. This filter
is designed to emulate the frequency responses of the human head
and pinnae when a sound source is approximately 150 degrees in
azimuth from the front of the listener. This type of frequency
response is called a "Head Related Transfer Function" or HRTF.
These frequency response functions have been measured for many
angles and for many different people. In general, there is a strong
notch in the frequency response at about 5 kHz when a sound source
is about 150 degrees from the front. A similar notch at about 8kHz
exists when a sound source is in front of a listener. Sound sources
to the side of the listener do not produce these notches. The
presence of the notch at 5 kHz is one of the ways in which the
human brain detects that a sound source is behind the listener.
The current standard for five channel sound reproduction recommends
that the two rear speakers be placed slightly behind the listener
at +/-110 or 120 degrees from the front. This speaker position
supplies good envelopment at low frequencies. However, listening
rooms often do not have a size or shape appropriate for placing
loudspeakers fully behind the listener and a side position is the
best that can be achieved. However, a sound generated to the side
of a listener does not produce the same level of excitement as a
sound that is generated fully behind a listener. In addition, film
directors often want a sound effect to come from behind the
listener, and not from the side.
The HRTF filter in the decoder adds the frequency notches of a rear
sound source so that a listener hears the sound as if it were
generated further behind the listener than the actual positions of
the loudspeakers. The filter is designed to vary with cs so that
the filter is maximum when cs is positive or zero, which causes
ambient sounds and reverberation to seem to be more behind the
listener. The filter is reduced as cs becomes negative and is
completely removed when cs is approximately -15 degrees. At this
point, the sound source appears to come fully from the side. The
filter is once again applied as cs goes further negative so that
the sound source appears to go behind the listener. The filter is
slightly modified to correspond to the HRTF function when cs is
fully to the rear.
30. Frequency Dependent Circuits: the Seven Channel Version
FIG. 3 shows the frequency dependent circuits in the seven channel
version of the decoder, which consisting of three sections.
However, the second two sections can be combined into one circuit.
The first two sections are identical to the two sections in the
five channel decoder, and perform the same function. The third
section is unique to the seven channel decoder. In version V1.11
and the November '96 application the side and rear channels had
separate matrix elements. The action of the elements was such that
the side and the rear outputs were identical, except for delay,
when cs was positive or neutral. The two outputs remained identical
until cs was more negative than 22 degrees. A s the steering moved
further to the rear, the side outputs were attenuated by 6 dB, and
the rear outputs were boosted by 2 dB. This caused the sound to
appear to move from the sides of the listener to the rear of the
listener.
In the present decoder, the differentiation between the side output
and the rear output is achieved by a variable shelf filter in the
side output. The third shelf filter in FIG. 3 has no attenuation
when cs is forward or zero. However, the zero in the shelf filter
moves rapidly toward 1100 Hz when cs becomes more negative than 22
degrees, resulting in an about 7 dB attenuation of the high
frequencies. Although this shelf filter has been described as a
filter separate from the shelf filter that provides the
"soundstage" function, the action of the two shelf filters can be
combined into a single shelf through suitable control
circuitry.
While the preferred embodiments of the invention have been
described and illustrated in this document, many other possible
embodiments exist. These and other modifications and variations
will be apparent to those skilled in the art without departing from
the spirit of the invention.
* * * * *